Imperial College London

Professor Norbert Hoffmann

Faculty of EngineeringDepartment of Mechanical Engineering

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n.hoffmann

 
 
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557City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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118 results found

Fontanela F, Grolet A, Salles L, Chabchoub A, Hoffmann Net al., 2017, Dark solitons, modulation instability and breathers in a chain of weakly nonlinear oscillators with cyclic symmetry, Journal of Sound and Vibration, ISSN: 0022-460X

© 2017 Elsevier Ltd. In the aerospace industry the trend for light-weight structures and the resulting complex dynamic behaviours currently challenge vibration engineers. In many cases, these light-weight structures deviate from linear behaviour, and complex nonlinear phenomena can be expected. We consider a cyclically symmetric system of coupled weakly nonlinear undamped oscillators that could be considered a minimal model for different cyclic and symmetric aerospace structures experiencing large deformations. The focus is on localised vibrations that arise from wave envelope modulation of travelling waves. For the defocussing parameter range of the approximative nonlinear evolution equation, we show the possible existence of dark solitons and discuss their characteristics. For the focussing parameter range, we characterise modulation instability and illustrate corresponding nonlinear breather dynamics. Furthermore, we show that for stronger nonlinearity or randomness in initial conditions, transient breather-type dynamics and decay into bright solitons appear. The findings suggest that significant vibration localisation may arise due to mechanisms of nonlinear modulation dynamics.

JOURNAL ARTICLE

Oberst S, Marburg S, Hoffmann N, 2017, Determining periodic orbits via nonlinear filtering and recurrence spectra in the presence of noise, Pages: 772-777

© 2017 The Authors. Published by Elsevier Ltd. In nonlinear dynamical systems the determination of stable and unstable periodic orbits as part of phase space prediction is problematic in particular if perturbed by noise. Fourier spectra of the time series or its autocorrelation function have shown to be of little use if the dynamic process is not strictly wide-sense stationary or if it is nonlinear. To locate unstable periodic orbits of a chaotic attractor in phase space the least stable eigenvalue can be determined by approximating locally the trajectory via linearisation. This approximation can be achieved by employing a Gaussian kernel estimator and minimising the summed up distances of the measured time series i.e. its estimated trajectory (e.g. via Levenberg-Marquardt). Noise poses a significant problem here. The application of the Wiener-Khinchin theorem to the time series in combination with recurrence plots, i.e. the Fourier transform of the recurrence times or rates, has been shown capable of detecting higher order dynamics (period-2 or period-3 orbits), which can fail using classical FouRiER-based methods. However little is known about its parameter sensitivity, e.g. with respect to the time delay, the embedding dimension or perturbations. Here we provide preliminary results on the application of the recurrence time spectrum by analysing the Hénon and the Rössler attractor. Results indicate that the combination of recurrence time spectra with a nonlinearly filtered plot of return times is able to estimate the unstable periodic orbits. Owing to the use of recurrence plot based measures the analysis is more robust against noise than the conventional Fourier transform.

CONFERENCE PAPER

Papangelo A, Ciavarella M, Hoffmann N, 2017, Subcritical bifurcation in a self-excited single-degree-of-freedom system with velocity weakening–strengthening friction law: analytical results and comparison with experiments, Nonlinear Dynamics, Pages: 1-10, ISSN: 0924-090X

© 2017 The Author(s) The dynamical behavior of a single-degree-of-freedom system that experiences friction-induced vibrations is studied with particular interest on the possibility of the so-called hard effect of a subcritical Hopf bifurcation, using a velocity weakening–strengthening friction law. The bifurcation diagram of the system is numerically evaluated using as bifurcation parameter the velocity of the belt. Analytical results are provided using standard linear stability analysis and nonlinear stability analysis to large perturbations. The former permits to identify the lowest belt velocity (Formula presented.) at which the full sliding solution is stable, the latter allows to estimate a priori the highest belt velocity at which large amplitude stick–slip vibrations exist. Together the two boundaries (Formula presented.) define the range where two equilibrium solutions coexist, i.e., a stable full sliding solution and a stable stick–slip limit cycle. The model is used to fit recent experimental observations.

JOURNAL ARTICLE

Papangelo A, Grolet A, Salles L, Hoffmann N, Ciavarella Met al., 2017, Snaking bifurcations in a self-excited oscillator chain with cyclic symmetry, COMMUNICATIONS IN NONLINEAR SCIENCE AND NUMERICAL SIMULATION, Vol: 44, Pages: 108-119, ISSN: 1007-5704

JOURNAL ARTICLE

Papangelo A, Hoffmann N, Ciavarella M, 2017, Load-separation curves for the contact of self-affine rough surfaces., Sci Rep, Vol: 7

There are two main approximate theories in the contact of rough solids: Greenwood-Williamson asperity theories (GW) and Persson theories. Neither of them has been fully assessed so far with respect to load-separation curves. Focusing on the most important case of low fractal dimension (D f  = 2.2) with extensive numerical studies we find that: (i) Persson's theory describes well the regime of intermediate pressures/contact area, but requires significant corrective factors: the latter depend also on upper wavevector cutoff of the roughness; hence, (ii) Persson's theory does not predict the correct functional dependence on magnification; (iii) asperity theories in the discrete version even neglecting interaction effects are more appropriate in the range of relatively large separations, also to take into consideration of the large scatter in actual realization of the surface.

JOURNAL ARTICLE

Grolet A, Hoffmann N, Thouverez F, Schwingshackl Cet al., 2016, Travelling and standing envelope solitons in discrete non-linear cyclic structures, MECHANICAL SYSTEMS AND SIGNAL PROCESSING, Vol: 81, Pages: 75-87, ISSN: 0888-3270

JOURNAL ARTICLE

Hadjihosseini A, Waechter M, Hoffmann NP, Peinke Jet al., 2016, Capturing rogue waves by multi-point statistics, NEW JOURNAL OF PHYSICS, Vol: 18, ISSN: 1367-2630

JOURNAL ARTICLE

Oberst S, Zhang Z, Campbell G, Morlock M, Lai JCS, Hoffmann Net al., 2016, Towards the understanding of hip squeak in total hip arthroplasty using analytical contact models with uncertainty, Pages: 5539-5549

© 2016, German Acoustical Society (DEGA). All rights reserved. Osteoarthritis in hip joints affects patients' quality of life such that often only costly orthopaedic surgeries i.e. total hip arthroplasty (THA) provide relief. Common implant materials are metal alloys, steel or titanium-based, plastics such as ultra-high molecular weight polyethylene, or biocompatible alumina and composite ceramics. Hard-on-hard (HoH) bearing articulations, i.e. ceramic-on-ceramic, or hard-on-soft combinations are used. HoH implants have been known to suffer from squeaking, a phenomenon commonly encountered in friction-induced self-excited vibrations. However, the frictional contact mechanics, its dynamics related to impingement, the effect of socket position, stem configuration, bearing size and patient characteristics are poorly understood. This study gives an overview of the state of the art biomechanical research related to squeaking in THA, with a focus on the effects of friction, stability, related wear and lubrication. An analytical model is proposed to study the onset of friction-induced vibrations in a simplified hemispherical hip stem rubbing in its bearing by varying the contact area. Preliminary results of the complex eigenvalue analysis and stick-slip motion analysis indicate that an increased contact fosters the development of instabilities, even at very small values of the friction coefficient owing to large local contact pressures.

CONFERENCE PAPER

Salles L, Staples B, Hoffmann N, Schwingshackl Cet al., 2016, Continuation techniques for analysis of whole aeroengine dynamics with imperfect bifurcations and isolated solutions, NONLINEAR DYNAMICS, Vol: 86, Pages: 1897-1911, ISSN: 0924-090X

JOURNAL ARTICLE

Akhmediev N, Soto-Crespo JM, Devine N, Hoffmann NPet al., 2015, Rogue wave spectra of the Sasa-Satsuma equation, PHYSICA D-NONLINEAR PHENOMENA, Vol: 294, Pages: 37-42, ISSN: 0167-2789

JOURNAL ARTICLE

Grolet A, Hoffmann NP, Schwingshackl C, 2015, Solitons in Non-Linear Cyclic Systems, Euromech Colloquium 573: Coupling and Nonlinear Interactions in Rotating Machinery

CONFERENCE PAPER

Kruse S, Tiedemann M, Zeumer B, Reuss P, Hetzler H, Hoffmann Net al., 2015, The influence of joints on friction induced vibration in brake squeal, JOURNAL OF SOUND AND VIBRATION, Vol: 340, Pages: 239-252, ISSN: 0022-460X

JOURNAL ARTICLE

Kruse S, Tiedemann M, Zeumer B, Reuss P, Hetzler H, Hoffmann Net al., 2015, The influence of joints on friction induced vibration in brake squeal (vol 340, pg 239, 2014), JOURNAL OF SOUND AND VIBRATION, Vol: 351, Pages: 311-312, ISSN: 0022-460X

JOURNAL ARTICLE

Peric R, Hoffmann N, Chabchou A, 2015, Initial wave breaking dynamics of Peregrine-type rogue waves: A numerical and experimental study, EUROPEAN JOURNAL OF MECHANICS B-FLUIDS, Vol: 49, Pages: 71-76, ISSN: 0997-7546

JOURNAL ARTICLE

Tiedemann M, Kruse S, Hoffmann N, 2015, Dominant damping effects in friction brake noise, vibration and harshness: the relevance of joints, PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART D-JOURNAL OF AUTOMOBILE ENGINEERING, Vol: 229, Pages: 728-734, ISSN: 0954-4070

JOURNAL ARTICLE

Akhmediev N, Soto-Crespo JM, Devine N, Hoffmann NPet al., 2014, Rogue waves of the Sasa-Satsuma equation in a chaotic wave field

CONFERENCE PAPER

Akhmediev N, Soto-Crespo JM, Devine N, Hoffmann NPet al., 2014, Rogue waves of the Sasa-Satsuma equation in a chaotic wave field

CONFERENCE PAPER

Chabchoub A, Kimmoun O, Branger H, Kharif C, Hoffmann N, Onorato M, Akhmediev Net al., 2014, Gray solitons on the surface of water, PHYSICAL REVIEW E, Vol: 89, ISSN: 2470-0045

JOURNAL ARTICLE

Chabchoub A, Kimmoun O, Branger H, Kharif C, Hoffmann N, Onorato M, Akhmediev Net al., 2014, Gray solitons on the surface of water, Physical Review E - Statistical, Nonlinear, and Soft Matter Physics, Vol: 89, ISSN: 1539-3755

The dynamics of surface gravity water waves can be described by the self-defocusing nonlinear Schrödinger equation. Recent observations of black solitons on the surface of water confirmed its validity for finite, below critical depth. The black soliton is a limiting case of a family of gray soliton solutions with finite amplitude depressions. Here, we report observations of gray solitons in water waves, thus, complementing our previous observations of black solitons. © 2014 American Physical Society.

JOURNAL ARTICLE

Chabchoub A, Peric R, Hoffmann NP, 2014, DYNAMICS OF UNSTABLE STOKES WAVES: A NUMERICAL AND EXPERIMENTAL STUDY, 33rd ASME International Conference on Ocean, Offshore and Arctic Engineering, Publisher: AMER SOC MECHANICAL ENGINEERS

CONFERENCE PAPER

Gräbner N, Tiedemann M, Von Wagner U, Hoffmann Net al., 2014, Nonlinearities in Friction Brake NVH - Experimental and Numerical Studies

Copyright © 2014 SAE International. Industry and academia agree that brake squeal is a nonlinear phenomenon. Consequently, using solely linear finite-element (FE) models and assessing the tendency of a brake system to squeal exclusively on the stability of the trivial solution is not appropriate. However, the latter approach - in the brake community known as complex eigenvalue analysis (CEA) - is extensively used in industry. Until now, nonlinear simulation approaches considering existence and stability of periodic solutions are mostly limited to minimal models. Among the variety of reasons for this the complexity of large-scale nonlinear models as well as the identification of nonlinear material and system parameters are crucial. This contribution discusses the relevance of nonlinearities in friction brake noise, vibration, harshness (NVH) and presents a novel simulation approach for brake squeal. On the basis of experimental results it is pointed out that the consideration of nonlinearities in simulation as well as new methods for numerical studies are necessary to cope with the system-inherent phenomena. After presenting experimental methods to characterize nonlinearities in brake systems, a novel simulation approach for brake squeal is introduced. This approach is based on subspace projection and allows the calculation of limit cycles considering nonlinearities such as joint damping. Studies are conducted for minimal models and a full-scale industrial FE model.

CONFERENCE PAPER

Hadjihosseini A, Peinke J, Hoffmann NP, 2014, Stochastic analysis of ocean wave states with and without rogue waves, NEW JOURNAL OF PHYSICS, Vol: 16, ISSN: 1367-2630

JOURNAL ARTICLE

Hoffmann NP, Bechthold J, Ziemer G, Evers K-U, Jochmann Pet al., 2014, Study of Local Ice Pressure Distribution on a Cylindrical Offshore Structure by Using Tactile Sensors., 22nd IAHR International Symposium on Ice

CONFERENCE PAPER

Kruse S, Hoffmann N, 2014, On the robustness of mode-coupling instabilities in friction induced systems, IEEE Global Communications Conference (GLOBECOM), Publisher: IEEE, Pages: 2939-2944, ISSN: 1930-529X

CONFERENCE PAPER

Kruse S, Stingl B, Hieke J, Papangelo A, Tiedemann M, Hoffmann N, Ciavarella Met al., 2014, The influence of loading conditions on the static coefficient of friction: A study on brake creep groan, Pages: 149-160, ISSN: 2191-5644

© The Society for Experimental Mechanics, Inc. 2014. This work focuses on the phenomenon of creep groan in brake systems as a problem leading to customer complaints (Kinkaid et al., J Sound Vib 267:105–166, 2003). It is potentially induced by a stick-slip phenomenon in the friction interface, but the dynamics are not well understood yet. This work tries to contribute to the understanding of the excitation mechanisms, the interaction of different length scales and possible measures to influence the phenomenon. It presents a dual—based on simulation and experiment—approach to investigate the phenomenon employing works of Ben-David and Fineberg (Phys Rev Lett 106, 2011) and Bar Sinai et al. (Geophys Res Lett 39, 2011). On the one side using the approach of Bar Sinai et al. (Geophys Res Lett 39, 2011) a one-dimensional simulation model is built up based on the contact state evolution of a representative brake pad and disc couple. This model is used to describe the interface dynamics while taking into account the surrounding simplified loading geometry. Parallel to this approach an experimental setup with a simplified disc brake is built up. As the simulation model allows to predict the level of the average static friction coefficient for different load cases, the simple test setup allows a validation. The experimental results show that the static friction coefficient can be systematically changed by modification of the loading geometry while one still observes the perturbing influence of uncontrolled process parameters.

CONFERENCE PAPER

Papangelo A, Stingl B, Hoffmann NP, Ciavarella Met al., 2014, A simple model for friction detachment at an interface of finite size mimicking Fineberg's experiments on uneven loading, PHYSICAL MESOMECHANICS, Vol: 17, Pages: 311-320, ISSN: 1029-9599

JOURNAL ARTICLE

Schwingshackl CW, Joannin C, Pesaresi L, Green JS, Hoffmann Net al., 2014, Test method development for nonlinear damping extraction of dovetail joints, ISSN: 2191-5644

© 2015, Conference Proceedings of the Society for Experimental Mechanics Series. All rights reserved. The traditional measurement techniques to acquire the linear dynamic response of a single component are well established and have been in use for many decades to provide reliable input data for model updating. The measurement of assembled structures normally follows a very similar approach, although the presence of joints can introduce a nonlinear dynamic behaviour, which impacts the measurement results. Applying traditional linear test methods to a highly nonlinear structure, such as a dovetail joint in an aircraft engine blade-disk connection, does not necessarily take the special features of the nonlinear response into account and may lead to unreliable data. This is particularly true, if modal information such as damping are required. The influence of the measurement setup and the test procedures must be well understood for an accurate measurement. In this paper the influence of the different measurement components on a simple clamped beam and a compressor blade dove tail test rig will be investigated. A particular focus will be on the support of the test rig, the location and control of the excitation and the influence of the accelerometer on the response. Based on the findings an approach will be recommended that allows a reliable measurement of the dynamic behaviour of this heavily nonlinear structure.

CONFERENCE PAPER

Soto-Crespo JM, Devine N, Hoffmann NP, Akhmediev Net al., 2014, Double peak rogue waves of the Sasa-Satsuma equation in a chaotic wave field

We study the properties of the chaotic wave fields generated in the frame of the Sasa-Satsuma equation, and analyze the presence of rogue waves in them. Our results also reveal the reason for the skewed profile of the exact rogue wave solutions. © 2014 OSA.

CONFERENCE PAPER

Soto-Crespo JM, Devine N, Hoffmann NP, Akhmediev Net al., 2014, Double peak rogue waves of the Sasa-Satsuma equation in a chaotic wave field

We study the properties of the chaotic wave fields generated in the frame of the Sasa-Satsuma equation, and analyze the presence of rogue waves in them. Our results also reveal the reason for the skewed profile of the exact rogue wave solutions. © 2014 OSA.

CONFERENCE PAPER

Tiedemann M, Kruse S, Hoffmann NP, 2014, Impact of joints on the dynamic behaviour of brake systems., Eurobrake 2014

CONFERENCE PAPER

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